The fast-growing aging population represents an important orthopedic market with a very specific need related to its low-quality cancelous or spongy bone (osteoporosis). Osteosynthesis procedures in such people are jeopardized by the risk of loosening in relation with the pullout or back-out of anchors in the bone. The reliability of an anchoring system depends on its ability to resist pulling out of the bone. Removal of an anchor may lead to extrusion, or even worse, loosening of any object attached to the bone. Known anchoring systems propose several solutions: Divergent or convergent screws have been proposed to oppose to pull out forces by increasing the grip. Locking mechanisms intended to secure the anchor within the object (such as locking screw or anti-reverse systems) avoid the pull out of the anchor but not the extrusion of the entire construct. Bicortical screwing may be dangerous and can make the construct too rigid. This can lead to the breakage of the implant itself. Special features like conical core, self-tapping profile and roughened surfaces of the anchor have been developed to increase the grip to the cancelous bone. Expandable mechanisms (such as threaded peg expanded with a coaxial inner member or “Molly screw”) crush and split fragile bone tissue and then creates an empty room around the buried part of the anchor. This can lead the construct to toggle and therefore to a condition for pull out or loosening in response to physiological micro-motions.
U.S. Pat. No. 6,695,844 to Bramlet et al discloses an expandable-winged fastener made up of an outer member and an inner mechanism able to protract or retract wings intended not only to increase the interface between the bone and the device but also to expand within the cancelous bone. Although the wings are blunted, the bone-implant interface is weakened as the wings expand or retract because the expanded wings broach through the bone as they pivot and therefore require the bone to remodel.
Other solutions use an interlocking mechanism, such as a K-wire which intersect a bone screw (“An Interlocking Screw for Fixation in Osteoporotic Bone” described by McKoy and al. in “internal fixation in osteoporotic bone” authored by Yuehuei H. An in 2002) or two members connected by their ends through a threaded connection (“Anchoring System for Fixing Object to bones” U.S. patent application Ser. No. 10/275,710 to Lionel Sevrain).
Connecting solutions address some of the problems, and provide increased strength and reliability, but require an aiming system that may represent a hurdle not only from a marketing standpoint but also from an anatomical standpoint. There is therefore a need for an improved anchoring system and installation method for anchoring an object to bone.
It would be desirable to provide a novel anchoring system for securing various objects to bones, such as spinal devices or instrumentations to the rachis, and to provide an anchoring system well adapted to prevent a pull out of the anchor and therefore the extrusion and/or the loosening of the object over time. It would also be desirable to provide an anchoring system to bring the needed reliability in such osteoporotic bone, without the hassle of an additional aiming guide.